1. Technical Field of the Invention
This invention generally relates to the production of microwave oven cooking elements useful both for food packaging, as well as in reusable utensils and specifically, to the production of microwave cooking elements which are capable of capturing and transferring microwave energy to the surface of a food item to be cooked in a microwave oven.
2. Description of the Background Art
The popularity of microwave ovens for cooking all or part of a meal has led to the development of a large number of food packages capable of cooking a food item in a microwave oven directly in the food package in which it is stored. The convenience of cooking food in its own package or a component thereof appeals to a large number of consumers. Further, many fast food restaurants are looking to fast, yet effective, ways of cooking and warming food which is less expensive than currently used methods. However, one dissatisfaction of microwave cooking for some foods is the inability to brown the food. It is often difficult to obtain grilling, browning and crisping of certain types of food in a microwave oven.
Microwave interactive films have been produced which are capable of generating heat at the food surface to crispen some food products. U.S. Pat. No. 4,883,936, issued to Maynard et al. and assigned to James River Corporation of Virginia, assignee of the present application, discloses the production of a microwave interactive heating element for food packaging which is selectively deactivated to provide an area or areas of microwave interactive material and an area or areas of deactivated material in a pattern on the surface of the heating element, so that only the area or areas having the interactive material untreated are fully capable of generating heat. Specifically, the patterned, deactivated heating element disclosed by Maynard et al. can be used to selectively brown the surface of a food item. Unfortunately, some food items, particularly very thick or solid foods, such as chicken fillets, absorb such a large portion of microwave radiation that the crisping element does not intercept sufficient energy for the desired browning and crisping at the surface of such a food item.
Other devices have also been developed to brown the surface of a food item in a microwave oven. U.S. Pat. No. 3,591,751, issued to Goltsos, discloses a browning apparatus for use in a microwave oven. Specifically, the apparatus includes microwave coupling devices located in contact or in close proximity to a food item for the purpose of browning. The coupling devices may be a plurality of metal rods supported on a dielectric board. The length of the rods themselves are integer multiples of a half wavelength with respect to the frequency of the microwave source to cause a resonant increase in the microwave currents on the surface of the rods. A separate apparatus may be used on both the top and the bottom of the food item to attempt to brown both sides thereof. However, using conventional single source microwave ovens in which the microwave source is located near the top of the oven cavity, more browning is observed on the top surface of the food than on the bottom surface of the food due to "shadowing" by the rods of the device on the top. A similar result in reverse holds true for microwave ovens in which the microwave source is located only near the bottom. Goltsos suggests providing two microwave source feeds located near the top and the bottom of the oven or a coupler to provide dual feeds. However, because conventional microwaves used by most consumers today only include a single microwave source near the top of the oven, this "shadowing" effect would occur while using the apparatus disclosed by Goltsos and therefore, would not be suitable for mass produced consumer use. Moreover, the apparatus of Goltsos is a large separate appliance type device and is, therefore, not contemplated to be used for food packaging.
U.S. Pat. No. 3,946,187, issued to MacMaster et al., discloses another example of a microwave browning or searing utensil for use in a microwave oven. The device is provided with a plurality of conductive metal members each of which are folded in such a manner to provide a continuous apex and two substantially equidistant legs. The legs are substantially one-quarter of a wavelength in height. Microwaves irradiated within the oven are converted by the array of conductive members to provide an intense fringing electric field in close proximity to a food item being heated thereon. The utensil may rest upon the floor of the oven cavity and may also be supported on top of a food load, as in Goltos et al. Again, however, while use of upper and lower utensils are suggested, there is no means for directing the microwave energy to both utensils disclosed in this patent, so the effects of "shadowing," discussed above, may still present a problem. Moreover, the device disclosed by MacMaster et al. is a separate utensil which is not designed to be disposable, as in popular microwave food packaging.
Devices have also been developed for providing uniform heating by microwave energy at desired points within an area of the microwave oven cavity. U.S. Pat. No. 3,271,552, issued to Krajewski, discloses a microwave heating apparatus which includes small antennas or supplemental radiating elements, which are preferably screwed into threaded holes provided in a portion of a wall of the microwave oven, to apply concentrated microwave energy to a food item. Krajewski also discloses the use of conductive strips which may be secured to and form a part of a food package. Specifically, the strips may be present as aluminum foil strips or rods. These elements do not, however, contact a food item nor provide browning or crisping thereof. Rather, the elements merely concentrate existing microwave energy which is present in the oven cavity.
Namiki et al. disclose in U.S. Pat. No. 4,992,636 a sealed container for microwave oven cooking wherein a lid is partially melted by microwave energy to form an opening therein. Specifically, the lid includes an antenna made of an electrically conductive material which concentrates microwave energy at a position near the front of the antenna and converts this energy to heat in order to melt a portion of the lid. However, the antenna does not provide a browning or crisping effect on food held within the container.
Some antennas have been developed which are useful for efficiently distributing heat within the interior of a food product, such as a turkey. U.S. Pat. No. 4,460,814, issued to Diesch et al., discloses an oven antenna probe for distributing energy in a microwave oven. Specifically, the antenna probe is designed to be inserted into a food item to distribute microwave energy within the food to provide adequate cooking inside and out. The antenna includes a source end antenna element which delivers power to a load end configured as a probe for insertion into the food. Several of the antenna-like structures may also be positioned throughout the oven cavity for reradiating energy towards a food product. The antennas do not, however, provide a sufficient amount of energy concentration to brown the surface of a food item, but rather redistribute the energy within the oven cavity to effectively cook a food item so that a similar amount of heating occurs at the center of a food item as at the outer portion of the food.
In addition, Keefer discloses in two U.S. Pat. Nos. 4,866,234 and 4,888,459, a microwave container which redistributes heat in a microwave oven to avoid "cold spots" which are commonly found within a microwave oven cavity. Specifically, the container may include a two-dimensional antenna or a slot antenna for receiving microwave energy in the oven cavity and to create a microwave field pattern or to act as a window for microwave energy, respectively. Again, these "antennas" do not provide a sufficient amount of concentrated or enhanced microwave energy near a food item to brown or grill the surface thereof.
Furthermore, U.S. Pat. No. 4,816,634 discloses a method and apparatus for measuring strong microwave electric field strengths. Moreover, this patent, as well as U.S. Pat. No. 4,934,829 disclose the use of cylindrical wave guides for cooking multi-component, layered food items. These disclosures are primarily directed to test probes or strips and do not provide a means of capturing and transferring energy in a microwave oven.
Consequently, a microwave oven heating device is needed which effectively captures microwave energy present in an oven cavity and transmits it to the surface of a food item which is conventionally browned or grilled. Further, a device is needed for heating or grilling food items in conventional, one source microwave ovens which can be included in disposable microwave food packaging or in reusable utensils.